DE 10 2014 016 076 A1 describes such a DC/DC converter assembly. The converter assembly transforms the voltage of a high-voltage (HV) battery to the level of a low-voltage (LV) battery in an electric or hybrid vehicle. The converter assembly also allows energy flow in the opposite direction from the LV battery to the HV battery.
The converter assembly includes first and second DC/DC converters. The first DC/DC converter is a series resonant switching converter. The series resonant switching converter is connected to the HV voltage level and is operated with fixed clocking. The series resonant switching converter thus has a fixed transmission ratio and generates a relatively low (i.e., intermediate) DC link voltage at its output. The voltage of the HV battery can vary. Due to the fixed transmission ratio, the DC link voltage varies when the voltage of the HV battery varies.
The second DC/DC converter receives the DC link voltage. The second DC/DC converter is a buck/boost converter. The buck/boost converter has a variable voltage ratio to the fixed voltage level of the LV battery.
On the LV level, in addition to loads that cannot be regenerated, such as an electric heater, there are loads such as electrical machines (motor/generator) which during load changes are capable of regenerative feedback. When these load changes happen dynamically, a large amount of energy is transferred back in a short time (ΔE=P*Δt). The greater the amount of energy ΔE and the shorter the time Δt, the higher the power P in the peak.
To catch such dynamic peaks, so-called super or ultra-capacitors are installed as an output capacitor on the LV level. A super or ultra-capacitor stores an amount of energy ΔE=½*C*ΔU2, where C is the capacitance of the capacitor and ΔU is a change in voltage of the capacitor. To keep both the voltage rise speed and the voltage swing low, these output capacitors are designed with capacities of several Farad. The installation space for such capacitors is on the order of the size of a shoe box. In addition, high-capacitance capacitors are expensive.